In the days and weeks since early May when his elderly parents were killed, Keith Dermond has been as puzzled as anyone else.

Russell and Shirley Dermond had been married 62 years. Both in their late 80s, they fell prey to unspeakable violence -- savagery as the Putnam County sheriff has called it -- and whoever inflicted it seems to have left nary a trace.

"It just eats at you that somebody could do this and possibly get away with it. ... We don't even know the motive at this point," Keith Dermond said by phone Wednesday. "It's very hard to catch somebody when you don't even know their motive, and you don't know whether they would do it again."

Dermond, 55, who lives in north Florida, said his family is still sorting through his parents' belongings.

"It's really tough," he said.

As the investigation into the couple's death reached its three-month mark, Sheriff Howard Sills said the case still weighs on him. Sills said he rose at 4 a.m. the other day to make note of an investigative angle that came to him in the night.

"Myself and one detective continue to work on this every day," the sheriff said.

Investigators this week polygraphed someone who came forward with information that could prove fruitful, "to see if they were truthful," Sills said.

On the sheriff's desk Wednesday were 4,000-plus pages of documents related to the Dermond couple's finances.

Russell Dermond, 88, had retired from corporate life -- much of it with clockmaking companies -- in the late 1980s to run a chain of Atlanta-area Hardee's restaurants. About 15 years ago, he and his wife, Shirley, 87, moved to Lake Oconee to live out their years in a golf-course community in a waterfront home.

Sometime during the first weekend in May, they were killed. Russell Dermond was decapitated. Neighbors concerned they hadn't seen him found his body May 6 in the garage of the Dermond's Carolyn Drive residence.

The 3,200-square-foot, $1 million home sits in a tree-shrouded cul-de-sac in the Great Waters subdivision, a dozen or so miles northeast of Eatonton.

Shirley Dermond's body didn't turn up until May 16. Fishermen found it in the lake, about five miles away by water.

Autopsies showed that she and her husband died of head wounds, but exactly how they were killed is anyone's guess.

The FBI and some Georgia sheriff's investigators have helped in the probe, poring over the case file.

Sills recently called on the expertise of two detectives from the Jacksonville Sheriff's Office's cold-case squad. Sills roomed with Jacksonville Sheriff John H. Rutherford when the two attended the FBI Academy.

The Florida detectives spent a day and a half in Eatonton examining crime-scene photographs and other materials, Sills said.

From the beginning of the investigation, authorities acknowledged that cracking the case might prove difficult.

No one reported hearing any commotion. The Dermonds likely were dead three days before Russell Dermond's body was found. Video cameras at the guard house at the entrance to the subdivision weren't working. And it is possible the killer came by boat. The 19,000-acre lake offered countless entry and escape routes.

While investigators for days scoured the Dermond house and areas around it for evidence, whatever they collected has yet to render a linchpin clue. And in the months since the killings, despite $55,000 in reward money, the flow of tips has all but dried up.

Even the kooks have quit calling.

"Reluctantly, or regretfully I guess is the better word," Sills said, "I have to admit that things have slowed considerably. We are not getting any new calls. The reward is not seeming to initiate any information. ... We're not even getting the psychics."

Keith Dermond said he and his family -- the elder Dermonds had two other children, a younger son and daughter -- appreciate the locals who contributed to the reward fund.

"It shows how much they were loved around the community," he said. "We're just hoping that maybe that'll trigger something ... that somebody will come forward and spill the beans."

On Wednesday, he was unaware of any imminent developments in the case.

"Not that we're losing hope, but it gets tougher and tougher to believe that something's gonna happen here," he said.

"I wouldn't say it's a cold case, but it's not looking too optimistic at this point. I'm sure the police and whatnot ... it's just driving them crazy."

Keith Dermond said he is confident Sills and other investigators are up to the task.

"We've heard from people around in the area that he certainly is somebody that is tenacious and won't give up. We're hoping that's true," he said.

"It's a very infamous case at this point so, yeah, I'm sure as far as his own reputation and everything else, they're doing everything they can to get it done."

He added that crime shows on television, the "CSI" types, make the unsolved killing of his mother and father all the more frustrating.

"The techniques they have of uncovering evidence and stuff like that," Keith Dermond said, "it's almost hard to believe that somebody could be that good a criminal that they didn't leave behind anything."

Copyright 2014 by The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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